Baxi Luna HT330 "foghorn ing" at low fire

BobT

This is a combo unit for heating and DHW. Had a contractor break the end of the vent (exhaust tip) of the unit and decided to replace it with an OEM replacement from Baxi. Thereafter I've been receiving Code 133 and the unit sounds like a foghorn at low fire, then flames out. High fire operates fine.
This unit is now 17 yrs old and may be ready for replacement but before I do, I'm seeking some input to repair it. I did change out the flame sensor as it was reading 1 microvolt vs the spec'd 3-5 with no change. Also reverted back to the original (broken) flue.
Other ideas would be welcome. I'm awaiting delivery of a flue analyzer to measure flue gasses. Unfortunately I do not have a QAA73 to change programming but if I need to procure one I will. There is no Baxi tech within 100 miles of Rochester NY that I'm aware of. Any ideas from the pro's??

pedmec

not sure what you mean by the exhaust tip but you changed the combustion process. you will need to perform a combustion test with a combustion analyzer.

BobT

Thanks, will do that...I suspected such but wondered if there was something else obvious I'd missed. ("Tip" being the external end where the diverter fitting is attached.) Tubes are clear and intact, vent pieces mated correctly, flame sensor replaced (it tested weak)

Tom_133

Im a little concerned that you may have dislodged the internal piping and you are getting some cross contamination from your exhaust into your fresh air. Thats a bit scary, make sure all internal fittings are good and sealed well, those silicone gaskets they use to seal definitely go bad. If you or anyone else is living in the space with that boiler, its smart to have someone with a combustion analyzer come check the intake port to make sure its clear of exhaust. Easy process, and could prevent a real problem, or someones death.